An electric vehicle (EV) charging system may be defined as a system for charging a high-voltage battery mounted in an EV using power of an energy storage device or a power grid of a commercial power source. The EV charging system may have various forms according to the type of EV. For example, the EV charging system may be classified as a conductive-type using a charging cable or a non-contact wireless power transfer (WPT)-type (also referred to as an “inductive-type”).
When charging an EV wirelessly, a reception coil in a vehicle assembly (VA) mounted in the EV forms an inductive resonant coupling with a transmission coil in a group assembly (GA) located in a charging station or a charging spot. Electric power is then transferred from the GA to the VA to charge the high-voltage battery of the EV through the inductive resonant coupling.
In a WPT system, an equivalent circuit of a transmission pad and a reception pad may be implemented by a transformer. The transformer implemented in this manner has a much larger air gap than usual converter-type transformers, and has a very low coupling coefficient. Since a leakage inductance Llk is relatively large as compared with a magnetization inductance Lm, it is difficult to transfer electric power to the output. Therefore, at least one capacitor is implemented in a primary pad and a secondary pad, i.e., the transmission pad and the reception pad, respectively, to resonate inductance of the primary and secondary pads and the at least one capacitor.
However, in the event of misalignment between the transmission pad and the reception pad, the magnetization inductance and the leakage inductance may fluctuate. This can cause a failure to reach resonance, thereby lowering a voltage and current transferred to the output and reducing overall efficiency.
Various types of coil structures have been proposed including a circular coil having a circular shape and a “DD” coil having two D-shaped coils. However, when the transmission and reception pads are composed of coils having different shapes (e.g., the circular coil for the transmission pad and the DD coil for the reception pad), a problem can occur in that the voltage and current transferred to the output decrease and the overall efficiency is reduced.